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What’s the explanation behind the bottom part of a candle flame being blue? I googled hard in vain. I read this. I don’t understand how it’s explained by the emission of excited molecular radicals in the flame. I read that a radical is a molecule or atom which has one unpaired electron. That made me more confused. I want a more detailed, clearer explanation.

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The red, orange, yellow, and white parts of a candle flame results from glowing soot. The color in this part of the flame is indicative of the temperature. The spectrum in this part of the flame is fairly close to that of a black body.

The blue part of the candle flame at the bottom of the flame results from chemiluminescence. Chemiluminescence is not black body radiation. The spectrum of that blue part of the flame has narrow peaks. It is nothing like the nice smooth curve of a black body. The color of that lowermost part of the flame is not indicative of temperature. The blue light is instead a byproduct of the chemical reactions taking place in that part of the flame, which is why the spectrum is so peaky.

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In the book "Physics of the Plasma Universe" Dr. Anthony Peratt puts candle flames near the bottom of "energy in electronvolts" portion of the 'plasma spectrum'. If you look at the chart below, you'll see candles flames about midway (ok, cosmologically) between the ends:

  • solar bodies and laser radiation
  • terrestrial flames
  • interstellar charged gases

the range of cosmological plasmas


I don't have the studies to do a discursive rendering of the chemical energetics, but you can see from this spectrum that most plasmas have fuel sources that are nearly-inexhaustible by comparison. Terrestrial flames have a very tight length-scales (relative to humans) before the dynamics become unsuitable (in terms of electrons per $cm^3$) to sustaining a plasma reaction — which produces the higher-energetic blues you see at the base of the flame.

You can see this more clearly when looking at a candle flame burning in space. The absence of gravity allows convection to very-evenly mix the soot and other combustibles, leading to a very clean plasma.


candle in space

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In a lit candle, when gaseous candle wax reacts with the oxygen in the air, the atoms will be unstably excited. To be stable, the excited electrons will relax to the ground state by emitting photons with energy equal to the energy difference between the 2 states. The photons’ energy doesn’t change much, so the wavelength doesn’t change much. The chemical reactions yield light with this spectrum. That light is blue to humans.

source – @gigacyan

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  • $\begingroup$ Your answer says that the flame (the visible result of a chemical reaction) is blue because the chemical reactions emit blue light. Not helpful. No mention of the part of a flame which is yellow, not blue, and why there is a difference. $\endgroup$ – sammy gerbil Sep 2 '16 at 15:11

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